Memory leak detection in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a memory leak in React Native?

A memory leak happens when the app keeps using memory that it no longer needs, causing the app to slow down or crash over time.

beginner

Why are memory leaks bad for mobile apps?

Because mobile devices have limited memory, leaks can make apps slow, use more battery, or crash, leading to a bad user experience.

intermediate

Name a common cause of memory leaks in React Native components.

Not cleaning up timers, subscriptions, or event listeners when a component unmounts can cause memory leaks.

intermediate

How can you detect memory leaks in a React Native app?

Use tools like React Native Debugger or Xcode Instruments to watch memory usage and find if memory keeps growing without releasing.

intermediate

What React hook helps prevent memory leaks related to side effects?

The useEffect hook with a cleanup function helps remove timers, subscriptions, or listeners when the component unmounts.

What happens if you forget to clear a timer in a React Native component?

A No effect on memory

B The app will automatically fix it

C The timer stops by itself

D Memory leak because the timer keeps running

If you don't clear timers, they keep running and hold memory, causing leaks.

Which tool can help you monitor memory usage in React Native?

A React Native Debugger

B Photoshop

C Excel

D Slack

React Native Debugger can show memory usage and help find leaks.

What should you do inside useEffect to avoid memory leaks?

A Return a cleanup function to remove side effects

B Do nothing

C Call setTimeout repeatedly

D Ignore component unmount

Returning a cleanup function removes timers or listeners when the component unmounts.

Memory leaks in mobile apps can cause:

A More storage space

B Faster app performance

C Slower app and crashes

D Better battery life

Leaks use memory unnecessarily, slowing the app and causing crashes.

Which of these is NOT a common source of memory leaks?

A Unremoved event listeners

B Properly cleaned up subscriptions

C Uncleared timers

D Retained references to unused objects

Proper cleanup prevents leaks; not cleaning up causes leaks.

Explain what a memory leak is in React Native and how you can prevent it.

Think about what happens when you forget to stop something running in your app.

Describe tools and methods you would use to detect and fix memory leaks in a React Native app.

Consider how you watch memory and clean up resources.

Practice

1.

What is a common cause of memory leaks in React Native apps?

useEffect hooks that do not clean up resources properly.

easy

A. Not cleaning up subscriptions or timers in useEffect

B. Using too many View components

C. Writing styles inline instead of in StyleSheet

D. Using functional components instead of class components

Solution

1. Step 1: Understand memory leaks in React Native

   Memory leaks often happen when resources like timers or subscriptions are not cleaned up after a component unmounts.

2. Step 2: Identify the role of useEffect cleanup

   The cleanup function inside useEffect is used to clear timers or unsubscribe to prevent leaks.

3. Final Answer:

   Not cleaning up subscriptions or timers in useEffect -> Option A

4. Quick Check:

   Failing to cleanup in useEffect causes leaks = B [OK]

Hint: Always clean timers and subscriptions in useEffect cleanup [OK]

Common Mistakes:

• Ignoring cleanup functions in useEffect
• Confusing styling issues with memory leaks
• Thinking functional components cause leaks

2.

Which of the following is the correct syntax to clean up a timer inside useEffect?

useEffect(() => {
  const timer = setTimeout(() => console.log('Hi'), 1000);
  return () => ???
}, []);

easy

A. clearTimeout(timer);

B. clearInterval(timer);

C. cancelTimeout(timer);

D. stopTimer(timer);

Solution

1. Step 1: Identify the timer type

   The timer is created with setTimeout, so it must be cleared with clearTimeout.

2. Step 2: Match cleanup function

   The cleanup function should call clearTimeout(timer) to cancel the timer.

3. Final Answer:

   clearTimeout(timer); -> Option A

4. Quick Check:

   setTimeout cleared by clearTimeout = A [OK]

Hint: Match setTimeout with clearTimeout in cleanup [OK]

Common Mistakes:

• Using clearInterval for setTimeout
• Using non-existent cancelTimeout or stopTimer
• Forgetting to return cleanup function

3.

What will happen if the following React Native component does NOT clean up the interval?

function Timer() {
  React.useEffect(() => {
    const id = setInterval(() => console.log('tick'), 1000);
  }, []);
  return null;
}

medium

A. The component will throw a syntax error

B. The interval stops automatically when component unmounts

C. The interval runs only once and stops

D. The interval keeps running even after component unmount, causing memory leak

Solution

1. Step 1: Analyze useEffect without cleanup

   The interval is set but no cleanup function is returned, so it never stops.

2. Step 2: Understand effect of missing cleanup

   When the component unmounts, the interval keeps running, causing a memory leak.

3. Final Answer:

   The interval keeps running even after component unmount, causing memory leak -> Option D

4. Quick Check:

   No cleanup means interval runs forever = C [OK]

Hint: No cleanup in useEffect means timers leak [OK]

Common Mistakes:

• Assuming intervals stop automatically
• Confusing syntax errors with runtime leaks
• Thinking interval runs only once

4.

Identify the bug causing a memory leak in this React Native component:

function DataFetcher() {
  React.useEffect(() => {
    const subscription = api.subscribe(data => console.log(data));
    // Missing cleanup function here
  }, []);
  return null;
}

medium

A. Empty dependency array in useEffect

B. Missing cleanup function to unsubscribe from api

C. Not returning JSX from component

D. Using arrow function inside useEffect

Solution

1. Step 1: Check subscription management

   The component subscribes to an API but does not unsubscribe on unmount.

2. Step 2: Identify missing cleanup

   Without returning a cleanup function that calls subscription.unsubscribe(), the subscription stays active causing a leak.

3. Final Answer:

   Missing cleanup function to unsubscribe from api -> Option B

4. Quick Check:

   Missing subscription cleanup causes leak = D [OK]

Hint: Always unsubscribe in useEffect cleanup [OK]

Common Mistakes:

• Thinking arrow functions cause leaks
• Confusing JSX return with memory leaks
• Ignoring cleanup for subscriptions

5.

You have a React Native component that fetches data every 5 seconds using setInterval. How should you modify the code to prevent memory leaks when the component unmounts?

function PollingComponent() {
  React.useEffect(() => {
    const id = setInterval(() => fetchData(), 5000);
    // What goes here?
  }, []);
  return null;
}

hard

A. Remove the dependency array from useEffect

B. Call fetchData() once outside useEffect

C. Return a cleanup function that calls clearInterval(id)

D. Use setTimeout instead of setInterval without cleanup

Solution

1. Step 1: Understand interval usage

   The component sets an interval to fetch data repeatedly every 5 seconds.

2. Step 2: Prevent memory leak on unmount

   To stop the interval when the component unmounts, return a cleanup function that calls clearInterval(id).

3. Final Answer:

   Return a cleanup function that calls clearInterval(id) -> Option C

4. Quick Check:

   Clear intervals in cleanup to avoid leaks = A [OK]

Hint: Always clear intervals in useEffect cleanup [OK]

Common Mistakes:

• Removing dependency array breaks effect logic
• Using setTimeout without cleanup causes leaks
• Calling fetchData only once misses polling